Are Imbalanced Drone Propellers Noisier? Maybe.

By Lauren Nagel

It is often assumed that imbalanced UAV propellers generate more noise than balanced ones, but the research suggests the relationship may be more complex. 

Several studies have investigated the impact of propeller imbalance on UAV noise levels, with mixed results depending on the type and extent of the imbalance. Factors such as tonal vs. broadband noise, propeller thickness, and UAV design all play a role in the observed acoustic effects. 

In this article, we review key studies on this topic and explore how different types of imbalance influence UAV noise production. Plus, a recommendation on how to reduce noise from drone propellers.

Table of Contents

1. Propeller Damage and Tonal Noise

2. Noise Signatures of Imbalanced Propellers

3. Standard vs. Low Noise UAV Propellers

4. How to Reduce Propeller Noise in UAVs

Figure 1: Quadcopter drone flying over natural landscape

1. Propeller Damage and Tonal Noise

Intuitively, one assumes that imbalanced propellers produce a greater amount of noise than balanced propellers. However, for small imbalances not significantly impeding the flight of the UAV, the results are mixed.

For example, in Pechan (2015), the authors observed the opposite effect. Propellers with various imperfections such as protuberances, notched blades, and mass imbalances were tested for noise production and rotated at various RPMs from 3000 to 5500 RPM.

In the majority of their experiments, the propellers with protuberances or notches showed, on average, equal or higher noise production (LZmax) compared to the undamaged propeller.

Figure 2: The highest detected noise level (LZmax) of damaged (B1 - B5), imbalanced (B6), and smooth propellers

In the case of the imbalanced propeller, which had material removed from one blade via sanding, the results showed that the balanced propeller actually produced more noise at higher frequencies at 5000 RPM. 

Figure 3: Noise spectra of smooth and imbalanced propellers at 3000 (left) and 5000 RPM (right)

The authors do not specify how much material was removed from the blade, so it is difficult to interpret these findings. The propellers that were damaged by addition or removal of material were also not characterized in detail, and likely represent an imbalanced state themselves.

The authors discuss the difference between tonal and broadband noise, which they differentiate as follows: “Tonal noise is related to the aerodynamics and kinematics of the blade in uniform flow, [...] thickness, loading and quadrupole noise. Broadband noise is more complex and is generated by the interaction between the flow and various components of the blade”. The tonal noise levels represent most of the contribution to the total noise, while the broadband noise represents only a small portion. They note that a major contributor to tonal noise is thickness noise, determined by the thickness of the propeller blade.

It may be hypothesized that by reducing the thickness of the imbalanced propeller’s blade through sanding, they may have reduced the thickness noise and lowered the overall tonal noise production.

2. Noise Signatures of Imbalanced Propellers

In another study by Iannace (2019), measurements of propeller noise were taken in an anechoic chamber using a quadcopter UAV with four 2-blade propellers. The noise was compared in three conditions: 1) unmodified propellers, 2) paper tape added to one blade of one of the propellers, 3) paper tape added to both blades of one of the propellers. The mass of the UAV, propellers, and paper tape were not indicated.

Four microphones were placed at different positions around the drone and linear sound pressure level was measured in dB. Interestingly, there were no significant differences in the sound measured in the three different conditions. The average sound level of the UAV with four balanced propellers was actually slightly higher (1.4 dB Lin) than the two imbalanced conditions.

Figure 4:Sound pressure level in dB Lin for each of the three conditions

Despite the very similar total noise levels, each of the conditions had a unique noise signature that the author’s neural network was able to differentiate with a success rate of over 97%.

3. Standard vs. Low Noise UAV Propellers

A third study performed by Semke (2020), sought to characterize how damage influences the acceleration, acoustic levels and flight performance of UAVs. Testing was performed using a DJI Phantom 4 Pro with four unique sets of propellers; Two of the sets had a low-noise design and the other two sets had a standard design. In each set, a centimeter of material was removed from the tip of one of the propellers to introduce an imbalance, while the other was unmodified.

The results demonstrated that for two of the standard blades, the imbalanced propellers showed a 4 dB noise level increase compared to the unmodified propellers, or a 5% increase. In contrast, the low-noise propellers showed little to no difference between the damaged and undamaged conditions.

Figure 5:Sound pressure level in dB of UAVs with undamaged and damaged propellers

They also studied the vibration of the aircraft using an on-board accelerometer. The results parallel the acoustic measurements: all damaged aircraft exhibited increased vibration, but the increase was less pronounced in UAVs with low-noise propellers.

Figure 6: Acceleration increase between UAVs with undamaged and damaged propellers

Each of these studies take for granted that the UAV’s propellers and motors were well balanced prior to the experiments. It would be interesting to see further studies of these effects with the following additions: 1) known propeller mass, 2) known mass or magnitude of the imbalance, 3) a measurement of the propeller’s balance against an established standard.

The takeaway from these studies is that small imbalances in UAV propellers are unlikely to significantly increase noise production, though large imbalances may result in more noise, especially as propeller size and RPM increases.

4. How to Reduce Propeller Noise in UAVs

Studies such as these bring up several questions about the relationship between propeller balancing and noise production.

The only sure way to find out is to characterize the propeller.

At Tyto Robotics, we have developed adynamic propeller balancing system that allows users to quantify and correct imbalances in their propellers.

The system pairs with a sound sensor to measure the noise production of your propeller at different states of imbalance.

Click here to learn more.

Figure 7:Adding weights to the blades of a propeller to achieve equal mass distribution

To learn more about how imbalanced propellers can impact your UAV, download our white paper: Consequences of Propeller Imbalance on UAV Performance:

Download White Paper